1. Field of the Invention
This invention pertains to a modem which may be operated in a half-duplex mode to exchange digital data over two-wire circuits without the need of a preamble prior to each data transmission. Therefore as soon as the modem stops transmitting it is instantaneously ready to receive data. The modem also includes transparent error detection means.
2. Description of the Prior Art
Conventional half-duplex modems are used to operate two-wire leased or dial telephone circuits. Two-wire circuits require transmission in only one direction at a time unless costly, non-ideal band splitting or echo cancellation techniques are employed. These non-ideal techniques induce impairments which necessarily limit the highest possible transmission rate.
Half-duplex modems which transmit data in only one direction at a time are not limited by these impairments. They can transmit at a higher data rate. At the highest possible data rate, complex receivers are required. These receivers have many adaptive functions, including gain control, carrier recovery, timing recovery, frequency offset compensation, adaptive automatic equalizers, and data randomizers. All of these functions require time to initialize which significantly delays the start of transmission.
Before transmitting data, a terminal first sends a Request-to-Send, (RTS), signal to its modem. Data transmission is not allowed, however, until the modem returns a Clear-to-Send (CTS) signal. The period between RTS and CTS is defined here as the CTS delay. During this time, special sequences are transmitted which allow the remote receiver to train all of its adaptive functions. These adaptive functions are reset prior to each transmission. As the modem's data rate increases, the CTS delay increases. For example, for a 2400 bps modem, the delay is 8 ms, for a 4800 bps modem the delay is 50 ms, and for a 9600 bps modem the delay is 253 ms.